1. Field of the Invention
The present invention relates to the growth of thin films, in particular to a method and apparatus for measuring and controlling accurately the optical thickness of a thin film during growth and to devices produced thereby.
2. Description of the Art
Thin film technology is increasingly relevant to a wide range of scientific and industrial fields including semiconductors, optics and telecommunications. One particular application of thin-film technology is in the field of fibre-optic telecommunications, where multi-layer thin-film optical filters are routinely used for wavelength division multiplexing (WDM). Wavelength division multiplexing (WDM) employs multiple optical signal channels simultaneously within a single optical fibre, each optical signal channel using light of a different wavelength. In wavelength division multiplexing, the thin-film optical filters are used to separate out (demultiplex) signals at the receiver.
The above filters effectively govern how much information can be passed down an optical fibre and there is an ever-present requirement to improve the data capacity of such WDM telecommunications systems by reducing the spacing between adjacent signal channels, thereby increasing the number of channels available. Conventional dense wavelength division multiplexing (DWDM) communications systems typically employ over one hundred optical signal channels, evenly spaced at 100 GHz intervals, in the range 186700 GHz to 197100 GHz according to the International Telecommunications Union (ITU) set of standardised frequencies [EXFO Electro-Optical Engineering Inc, Quebec City, Canada, Guide to WDM technology and testing, 2000, pp 15].
In an attempt to increase data capacity, the telecommunications industry is now proposing that the channel spacing be further reduced from 100 GHz to 50 GHz. However, such a proposal is not without attendant disadvantages and the reduced channel spacing of 50 GHz places onerous tolerance requirements on the multi-layer thin-film optical filters that are used to demultiplex the signals at the optical-fibre receiver. Consequently, the optical thickness of the individual thin films layers that comprise such a multi-layer filter have to be measured and controlled as precisely as possible during the growth process.
A conventional method for measuring and controlling the optical thickness of a thin-film during growth is to use a quartz-crystal monitor inserted into the growth chamber along with the optical filter substrate [Macleod, H. A. Thin-film optical filters. Bristol: Institute of Physics Publishing (IoP), 2001, chapter 11.3.2]
A quartz-crystal monitor is a quartz crystal that is induced to oscillate at a given frequency. The quartz-crystal monitor is placed in the growth chamber so that it is exposed to the same thin film growth as the substrate. In this way, the mass and hence natural frequency of oscillation of the quartz-crystal changes as material is deposited onto the crystal. The change in mass can be translated to physical thickness if the density of the material being deposited is known. However, in order for the optical quarter-wave thickness to be calculated, the refractive index of the thin film at the design wavelength must also be known. These material parameters may depend on the growth process parameters, so may not be known to sufficient accuracy prior to the growth. At present, there is no satisfactory way of measuring the refractive index of the thin-film during deposition.
Optical monitoring systems may also be used to measure and subsequently control the thin-film thickness during growth. Optical monitoring systems typically consist of some sort of light source illuminating a test substrate which may or may not be one of the filters in the production batch, and a detector analysing the reflected or transmitted light [Macleod, H. A. Thin-film optical filters. Bristol: Institute of Physics Publishing (IoP), 2001, chapter 11.3.2]
For example, when a DWDM filter is being grown, a laser, whose wavelength is set to be at the design wavelength of the filter, is transmitted through the substrate. The variation in laser light intensity is then measured as the filter grows and the position of the first peak (or trough) in the transmission characteristic is looked for. Growth must be stopped before the first peak (or trough) is passed. This method is sometimes referred to as a laser turning point method. This method is an improvement over the quartz-crystal monitor since no physical parameters of the thin film material are required. However, laser turning point methods suffer several disadvantages. Firstly, terminating the growth process at the first peak (or trough) is difficult to achieve accurately since the intensity of the transmitted laser light can only normally be measured to an accuracy of +/−5% and the rate of change in intensity is very low.
Also, there is no possibility with this method of checking that the true turning point has been reached.
The accuracy of such an optical monitoring system may be improved by simulating the desired optical filter characteristic prior to growth. Typically, a broadband source is used to illuminate the filter and the broadband transmission characteristic is monitored as the filter grows. Deposition is terminated when the difference between the measured transmission characteristic and the simulated transmission characteristic satisfies a pre-determined error criterion.
A hybrid monitoring system utilising a quartz crystal monitor and an optical turning point technique in combination may also improve the accuracy with which the thickness of the individual thin-films layers may be monitored.
Despite any improvements that may be derived from combining the above monitoring techniques, all of the aforementioned methods have another inherent drawback; namely the methods are real-time monitoring techniques which only provide an indication of when the design characteristic of the thin-film has been achieved. Furthermore, any latency between the monitoring system providing an indication that the desired thin-film thickness has been achieved and the cessation of the growth process may lead to unacceptable inaccuracies in the thin-film thickness, leading to a degradation in the performance of the thin-film. Alternatively, the above methods will have to stop the growth process early if there is a significant time to termination of growth after any stop command is given.